It's interesting that this comes up now, as a similar discussion recently took place on the program director listserv. Many programs have a pre- or post-interview social event, and a reasonable number of programs do not.
Most programs that have an event say they do this for a few reasons- 1) to give the applicant a chance to meet the residents, and therefore hopefully allow the applicant to decide whether they are a "fit", 2) to screen out the very few who make an absolute ass of themselves, and 3) some feel the pressure to do it because many other programs do it. The overwhelming reason is #1 above, to allow the applicants some unemcumbered time with the residents.
Of the programs that don't do this, they provide many reasons including- 1) It becomes expensive to host dinners every week for about 20 people, and they would rather use limited GME resources on educational programs (or the institution flat out refuses to pay for it), 2) they provide similar "alone" time for the residents and applicants to meet without faculty present during the actual interview day, 3) they feel that the planning/timing of interview season is difficult enough for applicants without burdening them to attend a night-before event, and 4) they don't want to ask residents to spend another night away from family and non-residency friends when they already have a very busy schedule.
To relate to this thread more directly, most PDs say that only about 50% of the applicants actually are able to make the event due to scheduling conflicts. There are certainly no hard feelings from the program side when applicants can't make these events- they are purely a "service" to the applicants to allow them some time to mingle and meet with residents and get a feel for the "personality" of the group. An exceedingly small number of people actually make an ass of themselves (although the stories are epic), so this is clearly not a primary reason for programs to have one of these events.
I would tell those of you who have trouble making these events that it is ok to miss them if it is a burden to your schedule or finances. If the program remains a potential top choice closer to the end of the season and you couldn't make the social, take a day to go back and see the residents and faculty in their work environment, see the interaction between residents and residents, between nurses/ancillary staff and residents, and between residents and faculty. You may spend less time and money doing this one or two times than you will keeping a hotel room an extra night on most of your interview days or broadening out your interview schedule to accomodate extra hours or days at each venue. If you can't make the night-before social but have a few hours free after the interview, consider emailing the PD ahead of time and ask if you can spend a few hours observing in the ED after your interview day. You will likely obtain pretty similar information to what you might get from the social event.
Short version- go if you can, but don't lose sleep if you aren't able to make it. If you couldn't make it and the program ends up in your top 2 or 3 (and/or you feel you didn't really get a good feel for the personality of the program), find a way to get back for a second visit. Just my 2 cents.